Add fixed version to phpunit used in travis (#2673)

tests.sh

@@ -477,7 +477,7 @@ build_php5.6_mac() {
   export PATH="$PHP_FOLDER/bin:$PATH"
 
   # Install phpunit
-  curl https://phar.phpunit.de/phpunit.phar -L -o phpunit.phar
+  curl https://phar.phpunit.de/phpunit-5.6.10.phar -L -o phpunit.phar
   chmod +x phpunit.phar
   sudo mv phpunit.phar /usr/local/bin/phpunit